Editing Cons (section)

{{short description|Function and primitive data structure in Lisp and other functional programming languages}}
{{about|the Lisp programming function}}
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In [[computer programming]], '''{{lisp2|cons}}''' ({{IPAc-en|ˈ|k|ɒ|n|z}} or {{IPAc-en|ˈ|k|ɒ|n|s}}) is a fundamental [[subroutine|function]] in most dialects of the [[Lisp (programming language)|Lisp]] programming language. {{lisp2|cons}} '''''cons'''tructs'' memory objects which hold two values or pointers to two values. These objects are referred to as (cons) cells, conses, non-atomic [[s-expressions]] ("NATSes"), or (cons) [[ordered pair|pairs]]. In Lisp jargon, the expression "to cons ''x'' onto ''y''" means to construct a new object with <code>(cons ''x'' ''y'')</code>. The resulting pair has a left half, referred to as the {{lisp2|car}} (the first element, or ''[[CAR and CDR|'''c'''ontents of the '''a'''ddress part of '''r'''egister]]''), and a right half, referred to as the {{lisp2|cdr}} (the second element, or ''[[CAR and CDR|'''c'''ontents of the '''d'''ecrement part of '''r'''egister]]'').

It is loosely related to the [[object-oriented programming|object-oriented]] notion of a [[constructor (computer science)|constructor]], which creates a new object given arguments, and more closely related to the constructor function of an [[algebraic data type]] system.

The word "cons" and expressions like "to cons onto" are also part of a more general [[functional programming]] jargon. Sometimes [[Operator (programming)|operators]] that have a similar purpose, especially in the context of list processing, are pronounced "cons". (A good example is the <code>::</code> operator in [[ML programming language|ML]], [[Scala (programming language)|Scala]], [[F Sharp (programming language)|F#]], [[Lean (proof assistant)|Lean]], [[Coq (software)|Coq]], and [[Elm (programming language)|Elm]] or the <code>:</code> operator in [[Haskell (programming language)|Haskell]], which adds an element to the beginning of a list.)